Copying lens system

ABSTRACT

A copying lens system of a three-unit/three-element composition that can be made from relatively inexpensive optical materials and which exhibits satisfactory performance at copying ratios ranging from about 0.5 to 2.0. The lens system includes, in order from the object side, a first lens of positive meniscus having a convex surface directed to the object, a negative second lens having a concave surface on both sides, and a positive third lens having a convex surface on both sides. The lens system satisfies the following conditions: 
     (1) 1.70&lt;n 1  +ν 1  /200≦1.93 
     (2) 0.10&lt;n 3  -n 1   
     (3) 0.06&lt;d t  /f t  0.11 
     (4) 0.60&lt;-r 3  /r 4  &lt;1.05 
     (5) 1.40&lt;-r 5  /r 6  &lt;2.70 
     where 
     f t  : the focal length of the overall system; 
     n i  : the refractive index of the ith lens at the d-line; 
     ν i  : the Abbe number of the i th  lens; 
     r j  : the radio of the curvature of the j th  lens; and 
     d t  : the overall length of the system.

BACKGROUND OF THE INVENTION

This application is based on and claims priority from JapaneseApplication No. Hei. 2-274778 filed Oct. 12, 1990, the disclosure ofwhich is incorporated herein.

FIELD OF THE INVENTION

The present invention relates to a copying lens system of athree-unit/three-element composition that exhibits good performance atcopying ratios ranging from about 0.5 to 2.0.

BACKGROUND

With the increasing demand for manufacturing smaller copying machines ata lower cost, the need has arisen to use smaller and less costly copyinglens systems. Two major approaches for realizing such a lens systems area symmetrical type of lens system having a four-unit/four-elementcomposition that has an F_(no). of about 5.6 as proposed in ExaminedJapanese Patent Publication No. 51091/1985, and an asymmetrical typelens system having a three-unit/three-element composition that uses theminimum necessary number of lens elements to obtain a flat image andthat has an F_(no). of 1:8 as proposed in Unexamined Published JapanesePatent Application No. 180924/1988.

However, the conventional copying lens system of athree-unit/three-element composition is difficult to manufacture at alow cost since the individual lens elements have high refractive indicesand, therefore, cannot be made of inexpensive optical materials. On theother hand, the conventional four-unit/four-element composition isdisadvantageous in that it is not compact.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a copying lens systemof a three-unit/three-element composition that can be made fromrelatively inexpensive optical materials and which exhibits satisfactoryperformance at copying ratios ranging from about 0.5 to 2.0.

This object of the present invention has been attained by a copying lenssystem of a three-unit/three-element composition which comprises, inorder from the object side, a first lens element of positive meniscushaving a convex surface directed to the object, a negative second lenshaving a concave surface on both sides, and a positive third lens havinga convex surface on both sides, which lens system satisfies thefollowing conditions:

(1) 1.70<n₁ +ν₁ /200≦1.93

(2) 0.10<n₃ -n₁

(3) 0.06<d_(t) /f_(t) <0.11

(4) 0.60<-r₃ /r₄ <1.05

(5) 1.40<-r₅ /r₆ <2.70

where

f_(t) : the focal length of the overall system;

n_(i) : the refractive index of the ith lens at the d-line;

ν_(i) : the Abbe number of the i^(th) lens;

r_(j) : the radio of the curvature of the j^(th) lens; and

d_(t) : the overall length of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of the copying lens systemaccording to Example 1 of the present invention;

FIG. 2 is a graph plotting the aberration curves obtained by the lenssystem of Example 1 at a copying ratio of 1.0;

FIG. 3 is a graph plotting the aberration curves obtained by the lenssystem of Example 1 at a copying ratio of 0.5;

FIG. 4 is a graph plotting the aberration curves obtained by the lenssystem of Example 1 at a copying ratio of 2.0;

FIG. 5 is a simplified cross-sectional view of the copying lens systemaccording to Example 2 of the present invention;

FIG. 6 is a graph plotting the aberration curves obtained by the lenssystem of Example 2 at a copying ratio of 1.0;

FIG. 7 is a graph plotting the aberration curves obtained by the lenssystem of Example 2 at a copying ratio of 0.5;

FIG. 8 is a graph plotting the aberration curves obtained by the lenssystem of Example 2 at a copying ratio of 2.0;

FIG. 9 is a simplified cross-sectional view of the copying lens systemaccording to Example 3 of the present invention;

FIG. 10 is a graph plotting the aberration curves obtained by the lenssystem of Example 3 at a copying ratio of 1.0;

FIG. 11 is a graph plotting the aberration curves obtained by the lenssystem of Example 3 at a copying ratio of 0.5;

FIG. 12 is a graph plotting the aberration curves obtained by the lenssystem of Example 3 at a copying ratio of 2.0;

FIG. 13 is a simplified cross-sectional view of the copying lens systemaccording to Example 4 of the present invention;

FIG. 14 is a graph plotting the aberration curves obtained by the lenssystem of Example 4 at a copying ratio of 1.0;

FIG. 15 is a graph plotting the aberration curves obtained by the lenssystem of Example 4 at a copying ratio of 0.5; and

FIG. 16 is a graph plotting the aberration curves obtained by the lenssystem of Example 4 at a copying ratio of 2.0.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The compositions of the copying lens systems according to four examplesof the present invention are described below with reference to drawingsand tables.

The copying lens systems according to the four examples are used forcopying at ratios ranging from about 0.5 (reduction) to 2.0(enlargement) including size-for-size, having a brightness on the orderof F_(no). 1:8.5 and capable of covering half-view angles of up to about22 degrees, with the focal length of the overall system being 100 mm.

FIGS. 1, 3, 5 and 9 show the lens arrangements of examples one throughfour, respectively, in which the first lens element L₁, the second lenselement L₂ and the third lens element L₃ are arranged in the statedorder from the object side. The first lens element L₁ is a positivemeniscus lens having a convex surface directed toward the object, thesecond lens element L₂ is a negative lens having a concave surface onboth sides, thereof and the third lens element L₃ is a positive lenshaving a convex surface on both sides thereof.

Graphs plotting the aberration curves obtained at copying ratios of 1.0,0.5 and 2.0 are shown in FIGS. 2-4 for Example 1; in FIGS. 6-8 forExample 2; in FIGS. 10-12 for Example 3; and in FIGS. 11-14 for Example4.

In Tables 1-4, which show the design specifications of Examples 1 to 4,r denotes the radius of curvature (in millimeters) of individual lenssurfaces, n signifies the refractive index of individual lenses, and νdenote their Abbe numbers.

Each of the copying lens systems according to Examples 1-4 of thepresent invention satisfies the following conditions:

(1) 1.70<n₁ +ν₁ /200 ≦1.93

(2) 0.10<n₃ -n₁

(3) 0.06<d_(t) /f_(t) <0.11

(4) 0.60<-r₃ /r₄ <1.05

(5) 1.40<-r₅ /r₆ <2.70

where

f_(t) : the focal length of the overall system;

n_(i) : the refractive index of the i^(th) lens at the d-line;

ν_(i) : the Abbe number of the i^(th) lens;

r_(j) : the radio of the curvature of the j^(th) lens; and

d_(t) : the overall length of the system.

The value of n₁ +ν₁ /200 in condition (1) relates to the refractiveindex and Abbe number of the first lens element. If this value exceedsits upper limit 1.93, the refractive index of the first lens elementincreases, which is favorable for the purpose of aberrationcompensation. However, as the refractive index increases, it becomesdifficult to manufacture the first lens element of inexpensive opticalmaterials, which is undesirable from an economic viewpoint. If the valuebecomes smaller than its lower limit, the refractive index of the firstlens element is too low to achieve effective compensation for variousaberrations.

The value of n₃ -n₁ in condition (2) is critical to ensure that the comathat occurs due to the reduced refractive index of the first lenselement can be effectively compensated by the third lens element. Ifthis value does not satisfy condition (2), effective compensation forthe coma cannot be achieved.

The value of d_(t) /f_(t) in condition (3) relates to the overall lengthof the lens system. If this value is smaller than its lower limit 0.06,a very compact system is realized but it becomes difficult to compensatefor various aberrations. In contrast, if this value exceeds its upperlimit 0.11, a condition favorable for copying performance will resultbut the overall size of the lens system is no less smaller than a systemof a four-unit/four-element composition and the compact requirementcannot be met.

The value of -r₃ /r₄ in condition (4) relates to the geometry of thesecond lens element. If this value exceeds its upper limit 1.05, theradius of curvature of the second surface of the second lens elementdecreases and it becomes difficult to suppress spherical aberration andcoma. Conversely, if this value becomes smaller than its lower limit0.60, not only is spherical aberration overcompensated but it alsobecomes difficult to achieve balanced compensation for astigmatism.

The value of -r₅ /r₆ in condition (5) relates to the geometry of thethird lens element. If this value exceeds its upper limit 2.70, the comadeveloping on the fourth lens surface will be undercompensated by thethird lens element. If the value is smaller than its lower limit,overcompensation for coma will occur in the third lens element.

Table 1 below shows the design specifications of the copying lens systemaccording to Example 1 of the present invention.

                  TABLE 1                                                         ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         17.222  4.232    1.56883                                                                             56.3   1.56883                               2         22.417  1.784    --    --     --                                    3        -24.982  1.017    1.58144                                                                             40.8   1.58144                               4         24.982  0.992    --    --     --                                    5         41.533  2.024    1.69350                                                                             53.2   1.69350                               6        -25.114  --       --    --     --                                    ______________________________________                                    

Table 2 below shows the design specifications of the copying lens systemaccording to Example 2 of the present invention.

                  TABLE 2                                                         ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         17.839  4.529    1.56883                                                                             56.3   1.56883                               2         42.516  1.671    --    --     --                                    3        -25.607  0.757    1.58144                                                                             40.8   1.58144                               4         26.375  1.234    --    --     --                                    5         50.742  1.991    1.69350                                                                             53.2   1.69350                               6        -24.956  --       --    --     --                                    ______________________________________                                    

Table 3 below shows the design specifications of the copying lens systemaccording to Example 3 of the present invention.

                  TABLE 3                                                         ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         18.674  4.776    1.60311                                                                             60.7   1.60311                               2         29.966  1.485    --    --     --                                    3        -28.638  0.613    1.58144                                                                             40.8   1.58144                               4         28.614  1.530    --    --     --                                    5         70.741  1.830    1.71300                                                                             53.8   1.71300                               6        -27.226  --       --    --     --                                    ______________________________________                                    

Table 4 below shows the design specifications of the copying lens systemaccording to Example 4 of the present invention.

                  TABLE 4                                                         ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         16.443  4.453    1.51633                                                                             64.1   1.51633                               2         20.855  1.799    --    --     --                                    3        -23.980  0.564    1.62004                                                                             36.3   1.62004                               4         23.980  0.656    --    --     --                                    5         35.612  2.306    1.74400                                                                             44.8   1.74400                               6        -25.046  --       --    --     --                                    ______________________________________                                    

Table 5 lists the values taken by the copying lens systems of Examples1-4 in connection with the above-discussed conditions (1)-(5).

                  TABLE 5                                                         ______________________________________                                        Condition   Ex. 1  Ex. 2       Ex. 3                                                                              Ex. 4                                     ______________________________________                                        (1)         1.85   1.85        1.91 1.84                                      (2)         0.12   0.12        0.11 0.23                                      (3)         0.10   0.10        0.10 0.10                                      (4)         1.00   0.97        1.00 1.00                                      (5)         1.65   2.03        2.60 1.42                                      ______________________________________                                    

Thus, each of the copying lens systems of Examples 1-4 satisfies thefollowing conditions of the present invention:

(1) 1.70<n₁ +ν₁ /200≦1.93

(2) 0.10<n₃ -n₁

(3) 0.06<d_(t) /f_(t) <0.11

(4) 0.60<-r₃ /r₄ <1.05

(5) 1.40<-r₅ /r₆ <2.70

As demonstrated by Examples 1-4, the present invention permits the useof low-index optical materials in manufacturing the first lens elementof a copying lens system of a three-unit/three-element compositioncontributing to a substantial reduction in the manufacturing cost. Inaddition, the copying lens system of the present invention exhibitssatisfactory performance at copying ratios over a wide range of 0.5-2.0.

Having the construction described above, the copying lens system of thepresent invention permits the use of inexpensive optical materials thathave somewhat lower refractive indices than those used in the first lenselement of the conventional version of a three-unit/three-elementcomposition. Accordingly, the lens system of the present invention iscompact and yet the cost of the first lens element can be reduced toabout one quarter to a tenth of heretofore necessary costs. Furthermore,this lens system exhibits satisfactory copying performance at ratiosover the wide range from 0.5 to 2.0.

What is claimed is:
 1. A copying lens system of athree-unit/three-element composition comprising, in order from theobject side, a first lens of positive meniscus having a convex surfacedirected to the object, a negative second lens having a concave surfaceon both sides, and a positive third lens having a convex surface on bothsides, which lens system satisfies the following conditions:(1) 1.70<n₁+ν₁ /200≦1.93 (2) 0.10<n₃ -n₁ (3) 0.06<d_(t) /f_(t) <0.11 (4) 0.60<-r₃/r₄ <1.05 (5) 1.40<-r₅ /r₆ <2.70wherein f_(t) : the focal length of theoverall system; n_(i) : the refractive index of the i_(th) lens at thed-line; ν_(i) : the Abbe number of the i^(th) lens; r_(j) : the radio ofthe curvature of the jth lens; and d_(t) : the overall length of thesystem.
 2. The lens system of claim 1 wherein said lenses satisfy thefollowing parameters:

    ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         17.222  4.232    1.56883                                                                             56.3   1.56883                               2         22.417  1.784    --    --     --                                    3        -24.982  1.017    1.58144                                                                             40.8   1.58144                               4         24.982  0.992    --    --     --                                    5         41.533  2.024    1.69350                                                                             53.2   1.69350                               6        -25.114  --       --    --     --                                    ______________________________________                                    


3. The lens system of claim 1, wherein said lenses satisfy the followingparameters:

    ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         17.839  4.529    1.56883                                                                             56.3   1.56883                               2         24.516  1.671    --    --     --                                    3        -25.607  0.757    1.58144                                                                             40.8   1.58144                               4         26.375  1.234    --    --     --                                    5         50.742  1.991    1.69350                                                                             53.2   1.69350                               6        -24.956  --       --    --     --                                    ______________________________________                                    


4. The lens system of claim 1 wherein said lenses satisfy the followingparameters:

    ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         18.674  4.776    1.60311                                                                             60.7   1.60311                               2         29.966  1.485    --    --     --                                    3        -28.638  0.613    1.58144                                                                             40.8   1.58144                               4         28.614  1.530    --    --     --                                    5         70.741  1.830    1.71300                                                                             53.8   1.71300                               6        -27.226  --       --    --     --                                    ______________________________________                                    


5. The lens system of claim 1 wherein said lenses satisfy the followingparameters:

    ______________________________________                                        Surface                                                                       No.      r        d        n.sub.3                                                                             ν   n.sub.1                               ______________________________________                                        1         16.443  4.453    1.51633                                                                             64.1   1.51633                               2         20.855  1.799    --    --     --                                    3        -23.980  0.564    1.62004                                                                             36.3   1.62004                               4         23.980  0.656    --    --     --                                    5         35.612  2.306    1.74400                                                                             44.8   1.74400                               6        -25.046  --       --    --     --                                    ______________________________________                                    